A micro electro mechanical system (MEMS) is configured by having mechanical parts, a sensor, an actuator, and an electronic circuit integrated together on the same substrate. The MEMS is used for various devices such as a printer head and a pressure sensor.
When the MEMS is used as a high-frequency device such as an impedance matching circuit and a high-frequency switch, losses can be suppressed.
Meanwhile, at the time of integrating mechanical parts and the like together on the same substrate, these mechanical parts can be easily integrated with electronic parts while using the same semiconductor manufacturing process. Therefore, a silicon substrate is used in many cases. When a cavity structure is formed on the silicon substrate, a movable unit is manufactured above the silicon substrate, and by performing an operation while using oscillation or deflection of the movable unit, characteristics of the MEMS can be obtained.
In the MEMS, to prevent the cavity structure formed on the silicon substrate from being crushed in a molding process and the like, the cavity structure is protected by a protection cap arranged above the silicon substrate. This protection cap is required with a gap between a pad formed on the silicon substrate and the protection cap to make it possible to connect the pad by wire bonding.
For example, Japanese Patent Application Laid-open No. 2007-222956 discloses the following method. A wiring layer is formed to surround a MEMS structure, and a silicon nitride film continuous from above the wiring layer to above the MEMS structure is formed. Plural openings are then formed on the silicon nitride film, and a cavity is formed between the semiconductor substrate and the silicon nitride film. The openings of the silicon nitride film are then closed, thereby air-tightly sealing the inside of the cavity.
However, when a MEMS is formed on a silicon substrate, a parasitic capacitance is generated between a wiring layer formed on the silicon substrate and the silicon substrate. Therefore, there is a problem that a high-frequency characteristic is degraded.
When a cavity structure is formed in the entire wiring region to reduce the parasitic capacitance between the wiring layer and the silicon substrate, the cavity structure is deviated from the protection cap, and this portion is crushed in a molding process and the like. Consequently, according to the conventional MEMS, a cavity structure cannot be provided at the outside of the protection cap. At the outside of the protection cap, a parasitic capacitance between the wiring layer and the silicon substrate cannot be reduced.